Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. About 50% of the refinery gasoline blending pool in the United States is produced by this process, with almost all being produced using the fluid catalytic cracking (FCC) process. Currently, all commercial FCC catalysts contain a crystalline aluminosilicate zeolite, particularly synthetic faujasite, i.e. Zeolite Y in an amorphous or amorphous/kaolin matrix of silica, alumina, silica-alumina, kaolin, clay or the like.
Numerous work has been done to increase thermal-steam (hydrothermal) stability of zeolites through the inclusion of rare earth ions or ammonium ions via ion-exchange techniques to lower soda content, which is destructive to Zeolite Y. Thermally and chemically modified Zeolite Y, such as ultrastable zeolite Y (USY) and calcined rare-earth exchanged Y zeolite (CREY) are used commercially to convert heavy hydrocarbon feedstocks into more valuable products.
In prior art fluid catalytic cracking catalysts, the active zeolitic component is incorporated into the microspheres of the catalyst by one of two general techniques. In one technique, the zeolitic component is crystallized and then incorporated into microspheres in a separate step. In the second technique, the in-situ technique, microspheres are first formed and the zeolitic component is then crystallized in the microspheres themselves to provide microspheres containing both zeolitic and non-zeolitic components.
It has long been recognized that for a fluid catalytic cracking catalyst to be commercially successful, it must have commercially acceptable activity, selectivity, and stability characteristics. It must be sufficiently active to give economically attractive yields, it must have good selectivity towards producing products that are desired and not producing products that are not desired, and it must be sufficiently hydrothermally stable and attrition resistant to have a commercially useful life.
U.S. Pat. No. 4,493,902, the teachings of which are incorporated herein by cross-reference, discloses novel fluid cracking catalysts comprising attrition-resistant, high zeolitic content, catalytically active microspheres containing more than about 40%, preferably 50-70% by weight Y faujasite and methods for making such catalysts by crystallizing more than about 40% sodium Y zeolite in porous microspheres composed of a mixture of two different forms of chemically reactive calcined clay, namely, metakaolin (kaolin calcined to undergo a strong endothermic reaction associated with dehydroxylation) and kaolin clay calcined under conditions more severe than those used to convert kaolin to metakaolin, i.e., kaolin clay calcined to undergo the characteristic kaolin exothermic reaction, sometimes referred to as the spinel form of calcined kaolin. In a preferred embodiment, the microspheres containing the two forms of calcined kaolin clay are immersed in an alkaline sodium silicate solution, which is heated, preferably until the maximum obtainable amount of Y faujasite is crystallized in the microspheres.
In practice of the '902 technology, the porous microspheres in which the zeolite is crystallized are preferably prepared by forming an aqueous slurry of powdered raw (hydrated) kaolin clay (Al2 O3:2SiO2:2H2O) and powdered calcined kaolin clay that has undergone the exotherm together with a minor amount of sodium silicate which acts as fluidizing agent for the slurry that is charged to a spray dryer to form microspheres and then functions to provide physical integrity to the components of the spray dried microspheres. The spray dried microspheres containing a mixture of hydrated kaolin clay and kaolin calcined to undergo the exotherm are then calcined under controlled conditions, less severe than those required to cause kaolin to undergo the exotherm, in order to dehydrate the hydrated kaolin clay portion of the microspheres and to effect its conversion into metakaolin, this resulting in microspheres containing the desired mixture of metakaolin, kaolin calcined to undergo the exotherm and sodium silicate binder. In illustrative examples of the '902 patent, about equal weights of hydrated clay and spinel are present in the spray dryer feed and the resulting calcined microspheres contain somewhat more clay that has undergone the exotherm than metakaolin. The '902 patent teaches that the calcined microspheres comprise about 30-60% by weight metakaolin and about 40-70% by weight kaolin characterized through its characteristic exotherm. A less preferred method described in the patent, involves spray drying a slurry containing a mixture of kaolin clay previously calcined to metakaolin condition and kaolin calcined to undergo the exotherm but without including any hydrated kaolin in the slurry, thus providing microspheres containing both metakaolin and kaolin calcined to undergo the exotherm directly, without calcining to convert hydrated kaolin to metakaolin.
In carrying out the invention described in the '902 patent, the microspheres composed of kaolin calcined to undergo the exotherm and metakaolin are reacted with a caustic enriched sodium silicate solution in the presence of a crystallization initiator (seeds) to convert silica and alumina in the microspheres into synthetic sodium faujasite (zeolite Y). The microspheres are separated from the sodium silicate mother liquor, ion-exchanged with rare earth, ammonium ions or both to form rare earth or various known stabilized forms of catalysts. The technology of the '902 patent provides means for achieving a desirable and unique combination of high zeolite content associated with high activity, good selectivity and thermal stability, as well as attrition-resistance.
The aforementioned technology has met widespread commercial success. Because of the availability of high zeolite content microspheres which are also attrition-resistant, custom designed catalysts are now available to oil refineries with specific performance goals, such as improved activity and/or selectivity without incurring costly mechanical redesigns. A significant portion of the FCC catalysts presently supplied to domestic and foreign oil refiners is based on this technology.
Catalysts which include phosphorous or phosphorous compounds have been described in U.S. Pat. Nos. 4,498,975, 4,504,382, 4,839,319, 5,110,776. These references disclose that the catalytic cracking activity and selectivity of zeolite catalysts may be improved by the addition of phosphorus.
For example, in accordance with the invention disclosed in U.S. Pat. No. 4,454,241, there is provided a catalyst comprising a crystalline alumino-silicate zeolite prepared from a clay starting material, a residue derived from said clay, and an effective amount of phosphorus, said catalyst having been prepared by the steps which comprise: (a) ion exchanging a clay derived alkali metal-containing Y-type crystalline aluminosilicate zeolite and the clay derived residue with a cation of a non-alkali metal to decrease the alkali metal content of said alkali metal-containing zeolite; (b) calcining the resulting ion exchanged zeolite and clay derived residue, and (c) contacting the resulting calcined zeolite and clay derived residue with a medium comprising an anion selected from the group consisting of dihydrogen phosphate anion, dihydrogen phosphite anion and mixtures thereof for a time sufficient to composite an effective amount of phosphorus with said calcined zeolite and residue.
U.S. Pat. No. 5,378,670 discloses the preparation of phosphorous modified zeolites/molecular sieves wherein a partially hydrogen, ammonium exchanged sodium zeolite/molecular sieve, is combined with a phosphorus compound, such as H3 PO4 to obtain a phosphorus-containing zeolite/molecular sieve composition that is thermally treated (steamed) to obtain a phosphorus reacted zeolite/molecular sieve that is subsequently reacted with additional phosphorus compounds to obtain a phosphorus treated zeolite/molecular sieve that contains from about 2 to 7 weight percent P2 O5. The steaming treatment is provided to yield an ultra-stable zeolite, having a reduced unit cell size relative to the starting material.